The invention discloses a water quality sampling unmanned catamaran and a sampling method thereof, and belongs to the technical field of environment monitoring. The water quality sampling unmanned catamaran comprises two catamaran bodies arranged in parallel, wherein the two catamaran bodies are in rigid connection through a fixed trapezoidal frame above the catamaran bodies; an upper platform anda lower platform are arranged on the trapezoidal frame; a cylindrical tube penetrating through the upper platform and the lower platform is arranged in the middle of the trapezoidal frame in a penetrating way; a windlass corresponding to the cylindrical tube is arranged on the upper platform; a bearing support frame matched with the windlass is also arranged on the upper platform; a water collecting pipe is wound on the windlass; one end, passing through the cylindrical tube, of the water collecting pipe is connected with a water sample collector; the other end of the water collecting pipe isconnected with a peristaltic pump fixed on the upper platform; and a water sample distributor is arranged on the lower platform, and is connected with the peristaltic pump. Through the arrangement ofa water sample collection device on the unmanned catamaran in parallel arrangement, the stability of the water sample collection device in the unmanned catamaran moving process is improved.